This invention relates generally to oscillators and more particularly to phase locked oscillators.
As is known in the art, there exists a need in many radio frequency applications such as in radar receivers and transmitters for highly stable oscillators or sources of radio frequency signals. One of the more general approaches is to use a so-called SAW stabilized oscillator in which a SAW device such as a delay line or a resonator is disposed within a feedback loop which has an integral number of 2.pi. radians of phase shift and excess small signal gain at a particular frequency within the passband frequency of the SAW stabilizing device.
In many of these applications, it is necessary to have an oscillator which is highly stable both over long periods of time, as well as, over relatively short periods of time. The first stability characteristic is generally referred to as aging characteristics of the oscillator and is principally related to long-term variations in the electrical characteristics of the devices providing the SAW stabilized oscillator. One of the major contributors to variations in oscillator performance over long periods of time may be the SAW device itself. SAW devices typically have aging rates on the order of several parts per million per year, although SAW resonators are now available having aging rates on the order of one part per million per year or less.
The second problem concerning frequency instability is short-term frequency variations or noise. SAW stabilized oscillators have relatively poor close to carrier phase noise characteristics. That is, there exists a relatively large amount of noise power per unit bandwidth at offset frequencies close to the carrier or fundamental frequency of the oscillator. This results principally from inherent characteristics of the SAW stabilizing element and other oscillator components such as the loop amplifiers.
It is known to address some of the aforementioned problems by including in the SAW oscillator loop a voltage controllable phase shifter which can introduce a relatively small amount of phase shift into the loop and thus adjust the frequency of oscillation of the loop or to lock the frequency of oscillation to a desired frequency. Generally to lock the frequency of the oscillator, an error signal is provided which represents the difference in phase between the fundamental frequency of the SAW oscillator loop and a reference signal.
When the reference signal has a frequency equal to the frequency of the oscillator loop, the above phase locking arrangement is relatively straight forward. Moreover, when the ratio of the fundamental frequency to the reference frequency is of the form of 2.sup.N where N is an integer .gtoreq.1, then the generation of an error signal is also relatively straight forward. For example, such a signal can be generated by using a cascade of frequency doublers to multiply the reference signal up to the frequency of the SAW oscillator. The frequency multiplied reference signal and SAW oscillator signal are fed to a phase detector to provide the error signal.
A problem arises, however, when the ratio of the fundamental frequency signal and the reference frequency signals has an odd prime factor or a ratio of the form 2.sup.N.multidot. m where N is an integer .gtoreq.1 and m is of the form (2n+1) where n is an integer .gtoreq.1. The problem is that known techniques for providing odd order frequency multiplication are not suitable for compact, low phase noise SAW stabilized oscillators. Two known techniques are to provide parametric frequency multiplication using non-linear devices such as step recovery diodes or varactor diodes. One problem with these techniques, however, is that they are relatively difficult to design with and, furthermore, the inherent characteristics of step recovery diodes and varactor diodes provide an odd order frequency multiplier circuit having unacceptable frequency stability. The poor frequency stability of the multiplier would provide a phase locking signal having likewise poor stability and thus degrade the overall performance of the phase locked oscillator.